def trim(s):
    if s == '':
        return s
    elif s[0] == ' ':
        return trim(s[1:])
    elif s[-1] == ' ':
        return trim(s[:-1])
    else:
        return s

if __name__ == '__main__':
    # 测试:
    if trim('hello  ') != 'hello':
        print('测试失败!')
    elif trim('  hello') != 'hello':
        print('测试失败!')
    elif trim('  hello  ') != 'hello':
        print('测试失败!')
    elif trim('  hello  world  ') != 'hello  world':
        print('测试失败!')
    elif trim('') != '':
        print('测试失败!')
    elif trim('    ') != '':
        print('测试失败!')
    else:
        print('测试成功!')